1. Field of the Invention
The invention relates generally to the field of direct-to-digital interferometry (spatial-heterodyne holography). More particularly, the invention relates to methods and machinery for obtaining two-wavelength differential-phase direct to digital interferograms (spatially-heterodyned holograms).
2. Discussion of the Related Art
The techniques and apparatus of basic direct to digital interferometry (holography) are well known to those of skill in the art.(1-2) A limitation of this technology is the difficulty of tracking the phase change in the object image when it involves multiple 2π steps. A 2π phase change occurs every time the optical object height changes by ½ of the laser wavelength. To obtain the full phase change of the object image, the multiple 2π's must be unwrapped. This unwrapping is often prone to errors, resulting in errors in the measured height of the object. In addition, if the height changes more than 2π over a distance less than the CCD pixel spacing at the object, the integral values of 2π of phase are completely lost (where 2π of phase shift occurs when the optical object height changes by ½ wavelength of the imaging laser beam for reflective imaging). To reduce the resulting errors, it is desirable to measure height variations at a much longer wavelength than that of the laser while still maintaining the lateral resolution of the shorter laser wavelength. This goal is accomplished in other forms of interferometry and digital holography by separately acquiring the phase data at two or more wavelengths and then looking at the difference of the phase measured by each wavelength.
The technique of using two wavelengths to measure large objects is well known in digital holography, holographic contouring and holographic interferometry.(3) In these techniques, phase information is obtained independently at two separate wavelengths. A digital hologram of an object at a first wavelength is obtained, and then a second digital hologram at a different wavelength is obtained. Each hologram is analyzed to obtain their individual phase and amplitude information. Finally, these two sets of phase data are then processed to obtain difference-phase data proportional to a scale length (i.e., the beat wavelength defined by the first wavelength and the second wavelength). Thus, the phase is measured at an effective wavelength much longer than either of the two probing wavelengths. In this way, height variations many times greater than the original laser wavelengths used have been measured.
A serious limitation of this known approach is that noise in each individual image is uncorrelated to the noise in the other image. When the difference between the two images is taken, the noise will not be reduced and is typically increased, thereby further reducing image quality.
Heretofore, the requirement of tracking the phase change in the object image when it involves multiple 2π steps without reducing image quality has not been fully met. What is needed is a solution that addresses this problem.